It is well known that the controlled dissociation of polyatomic molecules and recombination of the resulting active species in a low pressure, low temperature glow discharge is a feasible method for depositing films on the surface of a variety of substrates. It has been observed, however, that such films which are formed by use of the glow discharge system and methods known to date, exhibit either a slow rate of growth or several undesirable film properties, such as porosity and the inclusion of contaminant substances.
It is believed that such undesirable characteristics occur at least in part because reactant gases are either premixed or otherwise introduced together into the reaction chamber in a manner whereby both gases are subject to an exciting electric field.
It is known, for example, that during the deposition of silicon nitride using silane and nitrogen gases, because of residual oxygen in the system there is a spontaneous reaction between the activated oxygen and activated silane derivatives to form silicon oxide by-products and water vapor, both of which are detrimental to the silicon nitride film properties. Further, it is believed that complex polymerization reactions associated with silane occur in a glow discharge. The main volatile products of such polymerization reactions are di- and tri-silane derivatives and hydrogen, while nonstoichiometric subhydrides separate out as precipitates. Apparently the most important initial step in the discharge reaction of silane is the breaking of the Si-H bond with the formation of either or both a sylyl or sylene intermediate. The reaction mechanism leading to sylene formation is favored because of its exothermicity.
If the reactant gases are premixed or otherwise introduced into the reaction chamber, thereby being jointly subjected to an exciting electric field, adverse ion-molecule reactions may occur and lead to highly complex reaction products. In addition to common electron impact phenomena leading to the formation of ions, single collisions of, for example, fast Si atoms or ions (because of their low ionization potential) with silane molecules, are extremely likely. The ions produced in such reactions can have relatively low kinetic energies and might therefore be expected to undergo further chemical reactions.
One of the most likely reactions to occur in such environments is hydride-ion transfer reactions which have large rate constants in silane. The following reactions in the silane system can yield products containing two silicon atoms: EQU Si.sup.+ + SiH.sub.4 -- Si.sub.2 H.sub.2.sup.+ + H.sub.2 EQU siH.sup.+ SiH.sub.4 -- Si.sub.2 H.sub.3.sup.+ + H.sub.2 EQU siH.sub.3.sup.+ + SiH.sub.4 + Si.sub.2 H.sub.3.sup.+ + 2H
et cetera. These reactions are all exothermic and will be competing with nitrogen atoms for silane and its derivatives, thus inhibiting the full extent of silicon nitride formation. The possibility of foreign inclusion in the silicon nitride film of the various volatile by-products is also enhanced, thereby degrading subsequent film properties.